In recent years, in a highly accurate optical element such as an objective lens for an optical pickup apparatus, a diffractive structure is formed on its optical surface to improve an optical property by its diffractive effects. In this case, the diffractive structure is generally a microscopic structure of ring-shaped zones. When an objective lens is formed through injection molding with plastic as a material, it is necessary to process the mold to form a microscopic-groove form that corresponds to the structure of ring-shaped zones on its mold. As a material for the mold, cemented carbide is generally used, and this cemented carbide can be cut by a diamond cutting tool. Patent Literature 1 discloses a technology wherein a material of the mold is cut by a diamond cutting tool having a rectangular cutting face while the material is rotated, to form the microscopic-groove form.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2005-219185